Electrochemical method of producing white lead



Patented Jan. 20, 1 942 ELECTROCHEMICAL METHOD OF PRODUCING WHITE LEAD Gunnard E. Johnson, Hammond, Reginald G.

Bowman, Gary, and William J. Knox, Jr., Hammond, Ind., assignors to International Smelting and Refining Company, East Chicago, Ind., a corporation 0,! Montana No Drawing. Application December 6, 1937, I

I Serial No. 178,352

4 Claims.

This invention relates to the production of white lead and has for an object the provision of certain improvements in processes for producing white lead. More particularly, the invention contemplates the provision of certain improvements in processes for producing white lead by electrolysis.

The present-day commercial process for producing white lead electrolytically is carried out in a so-called bi-fluid cell having a lead anode and an iron cathode separated by a porous diaphragm. The anode is surrounded by an electrolyte (the anolyte) containing sodium acetate and a very small amount of sodium carbonate.

The cathode is surrounded by an electrolyte (the catholyte) containing sodium acetate and a relatively large amount of sodium carbonate. Each electrolyte is maintained in rapid circulation about its electrode. The circulation systems of the two electrolytes are entirely independent and no communication exists between the catholyte and anolyte in the cell except through the diaphragm in the cell.

In practice, a small amount of NazCO3, not exceeding about .05%, is added to the anolyte before placing the electrolytic cells in operation. When the current is turned on, the lead dissolves from the surface of the anode and is immediately precipitated as white lead at a slight h distance from the anode surface. The precipitate is carried down through the cell by the descending current of anolyte in a thick cloud without depositing on the anode surface. Hydrogen and the hydroxide ions are liberated at the cathode. The hydroxide and carbonate ions of the hydrolyzed sodium carbonate (NazCQal of the catholyte migrate through the diaphragm toward the anode, replenishing the anions removed from the anolyte by precipitation of the lead ions. By this electrolytic migration, the electro-chemical equivalent amount of anions required to precipitate the lead cations as they are dissolved at the anode are transported to the anolyte. Slight traces of carbonate can be detected in the anolyte but it is entirely free of dissolved lead salts.

' However, there is no sodium carbonate (Na-200a) present in the solution film in contact with the anode.

The foregoing outlines the major reaction, In practice, in order to replenish and conserve chemicals and to restore the loss in volume of catholyte due to decomposition of water at the cathode, evaporation and mechanical losses, anolyte is added to the catholyte in a small continuous stream. This introduces sodium acetate into the catholyte which is also acted upon by the current, the acetate ions replacing a part of the hydroxide and carbonate ions that migrate to the anolyte. This is compensated for by permitting a certain amount of seepage of catholyte into anolyte through the diaphragm. The seepage is controlled by selecting a diaphragm fabric of the desired porosity for a given hydrostatic solution head of catholyte of a selected composition in the cathode compartment. The addition of relatively small amounts of anolyte to catholyte and catholyte to anolyte permits adjusting the sodium concentration of the two electrolytes and the conservation of chemicals, since all make-up solution can then be added to the anolyte in the form of filtrate from the filters. All anolyte in' the filter cakes is replaced by water by a counter current washing operation and returned to the anolyte circulation system. In addition, wash water filtrate is also returned as required to replenish the losses enumerated above. As the volume of solutions transferred between the two electrolytes is very small the anolyte as previously stated is essentially free of the reacting salts and they are only brought into the zone where the white lead is precipitated in electro-chemical equivalent amounts, there is never an excess present to alter the reaction or influence the size of the particle.

The white lead which is removed continuously from the cell by the circulation of the anolyte, is removed from the anolyte by settling and filtering. The filter pulp is washed, dried, ground and air floated, and is then barrelled in a dry pulverulent form, The clear anolyte is recirculated through the cell.

Thecomposition-of the anolyte during. its passage through the cell remains unchanged except for a small increase in the acetate concentration. This is controlled by dilution with wash water from the filter. The catholyte is depleted in carbonate ions and a small percent of acetate ions, and enriched in hydroxide ions during its passage through the cathode compartment of the cell. The excess of hydrate ions is neutralized and the loss in carbonate ions restored by passing the catholyte through carbonation towers, in which a stream of descending solution passes through a rising stream of CO2 gas from a coke burning boiler. The acetate loss is replenished as noted by running a small stream of anolyte into the catholyte after it is discharged from the cells.

In the United States patent to Ralph M. Harrington, No. 1,308,948, it is recognized that the character and quality of the white lead product a may be controlled by regulating the amount of The composition of the basic lead'salt precipitated varies with the pH value of the solution in which it is formed. It will be apparent, therefore, that if only sodium carbonate (NazCOal and lead acetate (Pb(C2H3O2)2) were supplied to the solution, the sodium bicarbonate (NaHCOa) content would progressively increase and the ratio of H003 to OH ions in the resulting precipitated lead 'salt would increase-until finally only PbCOa would be produced. In other words the carbon dioxide (CO2) content of the precipitated white lead bears a direct relation to the sodium bicarbonate (NaHCOa) concentration of the anolyte. This has been found to be true in actual operation. In order to control the composition of the white lead produced, therefore, it is necessary to control the NaHCOa content of the anolyte. v

.Our discovery of the effect of th HCOa ion or bicarbonate content of the anolyte on the composition of the white lead produced is described in our United States Patent No. 1,845,713, dated February 16, 1932, which covers control of the bicarbonate content of the anolyte by maintain.- ing a uniform and deficient carbonation of the catholyte and adding an acid or a bicarbonate to the anolyte.

Our aforementioned patent also points out that it is desirable that the carbonate salt content of the anolyte be kept lowmuch lower than is required in order to avoid formation of white lead incrustations on the anode-in order to permit washing the white lead free of these salts in the finishing of the white lead separated from the anolyte. This is important in the commercial production of white lead by the electrolytic method, since the presence of any considerable amount of alkali metal carbonate in the white lead product renders it unfit for use as paint pigment. We have found that the difficulty in washing the white lead sufficiently free of adsorbed salts increases with increasing concentration of carbonate salt in the anolyte to a point at which it is practically impossible to eliminate them. This may be explained by adsorption of the salts in the white lead particles, but it seems more probable that a reaction may occur as follows: I p 2(2PbCO3.Pb(OI-I) 2) +Na2COa+NaHCO3= whereby the carbonate salt becomes chemically combined and therefore very diflicult or impos;

sible to remove.

bonate ion concentration of the anolyt at the value necessary for the production of a white lead product'of desired composition. The alkali metal hydroxide may be added to the anolyte in any suitable manner as, for example, by dissolving the hydroxide in the anolyte outside the cell, by adding a preformed solution of the hydroxide to the anolyte outside the cell, or by adding catholyte high in hydroxide (after electroly-' sis and prior to carbonation) to the anolyte outside the cell. Each of these procedures effects the desired control of the bicarbonate ion concentration in the anolyte, and hence the composition of the precipitated white lead, in a manner such that the addition of ions not entering into the composition of the precipitated white lead is not substantially greater than the loss of such ions in the course of normal operation of the cell. The solution of hydroxide for addition to the anolyte may be formed by employing as solvent the filtrate normally employed as makeup solution. The invention permits simplification of the'carbonation operation and provides a simple method of maintaining a supply of anolyte of the proper concentration with respect to bicarbonate. Instead of controlling the carbonation operation to completely or partially neutralize the catholyte solution from the cell (depleted in carbonate ion and high in hydroxide) and, in the case of the partially neutralized catholyte, adding an acid or bicarbonate to the anolyte to compensate for the incomplete neutralization, the carbonation operation may be permitted to proceed to the point at which excess carbonation is effected without the necessity for exercising extreme caution to secure exact neutralization or accurate deficient carbonation. The invention simplifies the process also by eliminating the necessity for exact control of addition of catholyte to anolyte by electrolysis and by seepage through the diaphragm to meet changes in operating conditions. The composition of anolyte thus harmfully altered can be adjusted readily by the method of the invention. In the preferred method of the invention, the carbonation operation is so controlled as to effect substantially uniform and excess carbonation of the catholyte for each particular set of operating conditions designed to produce a product of particular composition.

In accordance with our invention, using a bifluid cell with a lead anode suspended in an anolyte capable of yielding a solvent for the lead, e. g., sodium acetate, and a suitable inert cathode suspended in a catholyte containing sodium carbonate and separated from the anolyte by a suitable diaphragm, and to which an excess of carbon dioxide (CO is supplied in the carbonation operation, the anolyte solution is sampled and analyzed at regular intervals, say every hour, and its bicarbonate content adjusted to and maintained at the desired figure by the addition of an alkali metal hydroxide, as pointed out above.

For example, in a cell operated at a current density of 30 amperes per square foot of cathode area and producing approximately .011 pound of white lead per ampere hour and operating at an anolyte displacement rate of 18 gallons per minute while maintaining uniform and excess carbonation of the catholyte, the sodium bicarbone ate content of the anolyte is maintained at wash it suiliciently tree of such 'duce a first grade 2,270,783 0.0561% through adjustment by the addition of sodium hydroxide thereto to produce a white lead containing 11.60% 01' carbon dioxide (CO2).

content the acid carbonate content of the anolyte may be increased or decreased by the addition or lesser or greater amounts, respectively, of sodium hydroxide. The exact acid carbonate content re- 'quired for the production of a particular white lead product should be determined in each instance by experiment. The carbonate salt con- To produce white lead of greater or smaller CO2 tent of the anolyte is maintained-at not exceeding about twenty-five one-hundredths of one percent (0.25%), and preferably less than six one- 1 hundredths of one percent (0.06%). .The significance of the limited carbonate salt content of the anolyte specified may be amreciated by comparison with the limitation of the carbonate salt content of the anolyte necessary to avoid troublesome precipitation of white lead on the anodein accordance with the Harrington patent. In this case the carbonate salt content of the anolyte may run as high as six tenths of one percent (0.60%), which content, however, will produce white lead containing so much carbonate salt that it is not feasible in commercial tion to I as to prowhite lead paint pigment.

Weclaim: Y 1; In a process of producing white lead or constant particular composition by electrolysis in a bifluid cell involving the use of an anolyte capable under electrolysis ot-yielding a lead solvent and containing a bicarbonate and 01' a catholyte containing a carbonate which is altered during the course of the process by removal of carbonate ions, and involving carbonation of the catholyte to restore carbonate ions thereto and're-use of the regenerated catholyte, the composition of .the precipitated white lead being controlled by reference to the bicarbonate ion concentration in the anolyte, the improvement which comprises adjusting the bicarbonate ion concentration in the anolyte by subjecting the catholyte to ex-- cess carbonation and by incorporating hydroxyl ions in the anolyte in the form of a soluble metal hydroxide in such an amount that the 'amount of added metal ions of saidhydroxide is not substantially in excess of the amount of metal ions lost in the course of normal operation of the cell,

the amount of metal hydroxide incorporated in the anolyte being sufficient to establish therein bicarbonate ions in the desired concentration, whereby the addition of ions not entering into the composition oi! the precipitated. white lead is not substantially greater than the loss of such ions in the course of normal operation of the cell. 3

2. In a process of producing white lead of constant particular composition by electrolysis in a bifluid cell involving the use of an anolyte capable under electrolysis oi yielding a lead solvent and containing a bicarbonate and a catholyte containing a carbonate which is altered during the course of the process by removal of carbonate ions, and involving carbonation oi the catholyte to restore carbonate ions thereto and re-use of the regenerated catholyte, the compositlon of the precipitated white lead being controlled byreierence to the bicarbonate ion concentration in the anolyte, the improvement which comprises adjusting the bicarbonate ion concentration in the anolyte by subjecting the catholyte to excess carbonation and adding to the anolyte a sufllcient amount of uncarbonated catholyte containing hydroxyl ions to establish in the anolyte bicarbonate ions in the desired concentration, whereby addition of ions not entering into the composition of the precipitated white such ions in the course of normal operation of the cell. r

.lead is not substantially greater than the loss of 3. In the process of producing white lead of the amount of added alkali metal ions is not substantially in excess of the amount of alkali metal ions lost in the course of normal operation of the cell but suflicient to establish in the-anolyte bicarbonate ions in the desired concentration for precipitation of white lead of the particular composition.

4. In a process of producing white lead of constant particular composition by electrolysis in a bifluid cell involving the use of (1) an anolyte "capable under electrolysis of yielding a lead solvent and containing a carbonate and a bicarbonate and (2), a catholyte containing a carbonate which is altered during the course of the process by removal of carbonate ions, and involving carbonation of the catholyte to restore carbonate ions thereto and re-use of the regen-' erated catholyte, the composition of the precipitated white lead being controlled by reference to the bicarbonate ion concentration in the anolyte. the improvement which comprises maintaining the carbonate concentration in the anolyte at or.

below a predetermined maximum which is substantially less than the concentration resulting in objectionable deposition of white lead on the anode, and adjusting the bicarbonate ion concentration in the anolyte by subjecting a portion of the catholyte to excess carbonation and by adding to the anolyte the remaining portion of uncarbonated catholyte containing sodium hydroxide to establish in the anolyte bicarbonate ions in the desired concentration, whereby the concentration of ions not entering into the composition of the precipitated white lead is not increased in the course or normal operation of the cell.

GUNNARD E. JOHNSON.

REGINALD G. BOWMAN.

WILLIAM J. KNOX, JR. 

